Was on UA 979 last night (GRU-IAH). The PA system was down the entire flight. For the safety demo, the flight crew used megaphones. After that, it was radio silence all night until the final descent, when the crew came through row by row and gave the usual instructions about seat belts, upright, luggage stowed, etc. I must say it was a rather peaceful night! But it reminded me of an incident last year on an ExpressJet IAH-RSW when we returned to the gate and swapped aircraft due to a malfunctioning PA. The explanation the FA gave was that it was "against safety regulations" (or something like that) to fly without PA. So what's the real story here re no PA, to the extent there's any official protocol?